This application claims priority from Japanese Patent Application No. 200-45791 which is incorporated herein by reference.
The present invention relates to a method of manufacturing a Ti alloy poppet valve having a valve head with improved high-temperature strength.
Instead of an ordinary heat-resistant steel, a poppet valve is formed out of a Ti alloy which has a low specific gravity and a high specific strength to decrease the inertia mass of a valve operating mechanism and to increase the performance in an internal combustion engine.
The heat resistance temperature of an ordinary (xcex1+xcex2) alloy has an upper limit of about 500xc2x0 C., which is lower than that of heat resistant austenite steel. Accordingly, it would be difficult to apply an ordinary Ti alloy to an exhaust valve in which a working temperature is higher than that of the inlet valve.
To overcome this problem Japanese Patent Pub. No. 62-197610 discloses the molding of a valve head having heat resistance and a valve stem which has a lower thermal load from a high heat-resistant Ti alloy such as Ti-6Al-2Sn-4Zr-2Mo and an ordinary Ti alloy such as Ti-6Al-4, respectively. Japanese Utility Model Pub. No. 63-171604 discloses a valve stem made of heat-resistant steel combined with a valve head made of a high heat-resistant Ti alloy and a valve stem made of Ti alloy combined with a valve head made of heat-resistant steel.
However, when a high heat-resistant alloy is used in the valve head as above, the heat-resistance temperature of the valve head is forged and manufactured below its xcex2 transformation point to obtain ordinary hot forging or its regular xcex1 structure is limited to 600xc2x0 C. Thus, there is a disadvantage in fatigue and tensile strengths to use such a valve as an exhaust valve for a high-load engine in which the temperature of the valve head becomes about 800xc2x0 C.
In the conventional valve described above, the valve head and stem are made of different materials respectively, and the valve in which they are combined provides low tensile strength and reliability and increases the number of manufacturing steps, thereby increasing cost. Furthermore, a valve in which Ti alloy is connected to a heat-resistant steel could not lighten the valve.
In view of the disadvantages in the prior art, it is an object of the present invention to provide a method of manufacturing a Ti alloy poppet valve which is light and inexpensive so as to greatly improve the fatigue and tensile strengths which are required for a valve head under high temperature. The valve of the invention is made of a single alloy.
According to the present invention, there is provided a method of manufacturing a Ti alloy poppet valve, comprising the steps of: heating an end of a bar material made of an (xcex1+xcex2) or near xcex1 alloy which comprises a regular xcex1 structure at a temperature above its xcex2 transformation point to make an enlarged portion; and
forming said enlarged portion by hot forging into a valve head having an acicular xcex1 structure.
The valve head is easily formed from the acicular xcex1 structure to increase fatigue and tensile strengths under high temperature. Compared with a conventional valve in which a valve stem is welded to a valve head, the valve of the invention is made of a single alloy. The valve of the present invention thereby provides higher strength, lighter weight and simplification of manufacturing steps.